XIV 



THE CONTINUANCE OF LIFE 



305 



phyte or hydroid sometimes buds off and liberates sexual 

 swimming bells or medusoids, whose fertilised ova 

 develop into embryos which settle down and grow into 

 hydroids. This is perhaps the simplest and clearest 

 illustration of alternation of generations the alternate 

 occurrence in one life-history of two (or more) different 

 forms differently 

 produced. 



In autumn the 

 freshwater sponge 

 (Spongilla) begins 

 to suffer from the 

 cold and the scar- 

 city of food. It 

 dies away ; but 

 some of the cells 

 become compacted 

 together to form 

 " gemmules ' from 

 which in spring 

 male and female 

 sponges are de- 

 veloped. The males 

 are short-lived, but 

 their spermatozoa 

 fertilise the ova. of 

 the females. The 

 fertilised ovum de- 

 velops into a 

 ciliated e m b r y o, 



and this into the FIG. lOO. DIAGRAM OF 

 asexual sponge, 

 which produces the 

 gemmules. 



The large free-swimming and sexual jellyfishes of the 

 genus Aurelia produce ova and spermatozoa ; from the 

 fertilised ovum an embryo develops not into a jellyfish, 

 but into a sessile polyp-stage (the Hydra-tuba). This 

 grows and divides transversely into a strobila or " pile-of- 

 saucers " stage. " Saucer " after " saucer " is separated 



21 



A HYDROID COLONY, 

 SOME OF THE INDIVIDUALS OF WHICH HAVE 

 BEEN MODIFIED AS S\VIMM ING-BELLS OR 

 MEDUSOIDS ; ONE OF THESE HAS BEEN 

 LIBERATED. 



